The present invention relates to a method of operating a proximity sensor and also a proximity sensor that interacts with an initiator or trigger. The sensor has a tuned circuit and a frequency generator. A component that is independent of the material of the initiator is determined from a complex system variable that depends on a position and the material of the initiator.
Known proximity sensors contain a tuned circuit with a capacitor and a coil, whose impedance changes as a metallic initiator or target approaches. In the case of an inductive proximity sensor, the inductance of the tuned circuit coil is influenced by the initiator, but in the case of a capacitive proximity sensor, it is the capacitance of the tuned circuit capacitor that is influenced. As a result of the change in the impedance of the tuned circuit, an amplitude of the tuned circuit signal changes. The signal is rectified and, in the case of a proximity switch, is converted by a discriminator into a signal indicating the presence or absence of the initiator.
The tuned circuit amplitude depends on the tuned circuit frequency, on the position of the initiator, that is to say its distance from the sensor, and the material of the initiator. In the case of different initiators, the discriminator will generally respond at different switching distances, which is to say at a different distance between the initiator and the sensor. For this reason, commercially available proximity switches are initiator-material specific, and reduction factors in the switching distance are defined. For example, in the case of inductive proximity switches, the switching distance for a copper target is only 30% of the switching distance of a tool steel target, primarily because of the different magnetic properties.
Published, European Patent Application EP 0 288 921 A2 describes an inductive proximity switch which responds at the same switching distance for two different initiator materials. In this case, a differentiation is only made between non-ferrous metals such as copper or aluminium and ferromagnetic ferrous metals. Use is made of the fact that, for a desired switching distance, the impedance/frequency characteristics for the two types of initiator intersect at a point. This point determines both the tuned circuit frequency and the critical tuned circuit impedance to be detected by the discriminator. However, the independence of the initiator is merely ensured for the envisaged switching distance.
German utility model DE 94 12 765 U describes an inductive proximity sensor in which a variable that is substantially independent of the material of the initiator is obtained from a tuned circuit variable and the measured resonant frequency by forming a difference.
It is accordingly an object of the invention to provide a proximity sensor and a method for operating the proximity sensor which overcome the above-mentioned disadvantages of the prior art methods and devices of this general type, which is independent of the material of an initiator.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of operating a proximity sensor that interacts with an initiator. The proximity sensor has a tuned circuit and a frequency generator. The method includes determining a component from a complex system variable. The component is independent of a material of the initiator, and the complex system variable depends on a position and the material of the initiator. The determining step includes carrying out a determination of the component in a complex numerical plane by projecting the complex system variable onto a direction perpendicular to a straight line approximating a material dependence of the complex system variable.
The nub of the invention is to split off a component that is independent of the material of the initiator from a complex system variable, which depends on the position and the material of the initiator. The corresponding projection in a complex numerical plane is based on the finding that the dependence of the complex system variable on the material can be approximated by straight lines that are parallel for various initiator positions.
The system variables considered are, for example, the impedance of a tuned circuit belonging to the proximity sensor, the impedance of a tuned circuit coil, the amplitude of the tuned circuit signal or a voltage divider ratio between the tuned circuit and a series resistance.
According to a preferred embodiment of the invention, the aforementioned projection direction is determined numerically or experimentally, by different initiators being brought successively into the same position relative to the proximity sensor, the selected system variable being determined for each initiator and the system variable values determined in this way being linearly interpolated.
According to a further preferred embodiment, in order to reduce the influence of temperature fluctuations on the system variable, a second tuned circuit is provided as a comparative reference.
One advantage of the invention resides in the fact that a continuous signal, dependent only on the position of the initiator, is obtained. In the event of use as a proximity switch, it is therefore possible for the switching distance or if the discriminator threshold to be selected as desired.
In accordance with an added mode of the invention, there is the step of forming the straight line as a linear interpolation to at least two system variable values determined for any desired position of the initiator and different initiator materials.
In accordance with an additional mode of the invention, there is the step of forming the proximity sensor as an inductive proximity sensor and the complex system variable is an impedance of a sensor coil.
In accordance with another mode of the invention, the complex system variable is a tuned circuit impedance, a tuned circuit amplitude or a voltage divider ratio between the tuned circuit and a series resistor.
In accordance with a further mode of the invention, there is the step of forming the complex system variable as a difference between a tuned circuit amplitude or a tuned circuit impedance and a reference amplitude or a reference impedance that is independent of the initiator.
In accordance with a further added mode of the invention, there is the step of using a reference tuned circuit that is identical to the tuned circuit for supplying one of a reference amplitude and a reference impedance.
In accordance with a further additional mode of the invention, there is the step of using an evaluation circuit for converting a periodic signal, that depends on the position and the material of the initiator, a periodic signal of the frequency generator, and a phase angle into a DC signal.
With the foregoing and other objects in view there is provided, in accordance with the invention, a proximity sensor for interacting with an initiator. The sensor includes a tuned circuit, a frequency generator, and an evaluation circuit for determining a component, being independent of a material of the initiator, from a complex system variable being independent of the material and a position of the initiator. The evaluation circuit is connected to the frequency generator and coupled to the tuned circuit. A device is provided for projecting the complex system variable in a complex numerical plane onto a direction perpendicular to a straight line that approximates a material dependence of the complex system variable.
In accordance with a concomitant feature of the invention, the evaluation circuit has a phase shifter, a multiplier connected to the phase shifter, and a low-pass filter connected to the multiplier.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a proximity sensor and a method for operating the proximity sensor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.